This project is concerned with the Kamin "blocking" effect in Pavlovian fear conditioning. It involves conditioned suppression methodology, and it explores some variations of Kamin's 2-stage paradigm. It first attempts to find some direct evidence that excitatory apparatus cues block conditioning to discrete CSs, and it explores in some detail an obtained failure of an "artificial" apparatus cue to block. This exploration has led to a study of a number of variations of the Kamin procedure. For example, we have sought to determine whether blocking occurs when Cs durations are variable from trial to trial so that the time of US occurrence is unpredictable. We have also sought to determine whether blocking occurs in trace conditioning procedures, and in serial compound procedures. The general theoretical question that we are trying to answer is whether certain of our results can be best handled by the discrepancy Model of Rescorla and Wagner or whether they demand some notion of "surprise". We are assuming here that the notion of surprise may not have been fully captured by the Rescorla-Wagner concept of discrepancy.